Vladimir Kolesnikov

Like any other software WordPress does have bugs and security vulnerabilities. They get fixed when the developers become aware of them. 
However, sometimes several WordPress releases are vulnerable. The problem is that the developers do not backport security updates to older WordPress releases. 
As a result, those who do not want to upgrade their WordPress installations (for example, because newer WP releases tend to consume more system resources and 
upgrade may result in having to use more expensive hosting) remain vulnerable.
ICS Security Fixes is the plugin for those who do not upgrade their WordPress: the plugin tries to fix known WordPress vulnerabilities 
at that remaining compatible with as many old WordPress releases as possible. 
And though it cannot fix all known vulnerabilities, it makes older WordPress installations more secure.
<h3>Warning</h3>
This plugins requires that PHP 5 be installed. PHP 4 is not supported.

ICS Security Fixes

The plugin tries to fix known WordPress vulnerabilities for older WordPress versions. Warning: For PHP 5 only.

ICS Security Fixes vs WPVulnerability